The invention relates to a method for tempering a screw-type vacuum pump. Moreover, the invention relates to a screw-type vacuum pump suited for implementing said method.
From DE-A-198 20 523 a screw-type vacuum pump of the here affected kind is known. The multitude of heat problems has been disclosed. Cooling of the rotors revolving in a pump chamber involves special difficulties when the threads of the rotors exhibit a pitch which decreases from the intake side to the delivery side, frequently even also in combination with an increase in the width of the thread ridges. Rotors of this kind are subjected during operation to severe thermal stresses, in particular in the area of their delivery side, since the compression of the pumped gases produces a not insignificant amount of heat. Since the quality of a screw-type vacuum pump depends significantly on the gap between the rotors and the pump chamber housing, the manufacturers strive to keep this gap very small. However, opposed to this aim is the thermal expansion of the thermally highly stressed areas, rotors and housing. The pump chamber housing does not, or only slightly, take part in the thermal expansion of the rotors. A sufficiently large gap must be present. It was previously only in this manner possible to prevent the rotors from making contact with the housing with the attendant risk of standstill seizing. The problem detailed grows to be particularly grave when the rotors and the housing consist of different materials. In the instance of the coefficient of expansion of the housing being smaller than the expansion of coefficient of the rotor material (for example, housing made of cast iron, rotors of aluminium) there exists the risk of the rotors running against the housing. If the reverse expansion conditions exist, the pump's gap can increase such that the performance of the pump decreases.
It is the task of the present invention to design and be able to operate a screw-type vacuum pump of the here affected kind such that during thermal stresses its properties will not change substantially.